1. Field of the Invention
The present invention relates to a controller for drivingly controlling a servomotor in a machine tool, and more particularly, to a controller configured so that acceleration detecting means is provided on the machine tool to feed back acceleration information, to thereby stabilize an operation of the machine tool.
2. Description of Related Art
Normally, position, velocity, and current feedback controls are performed in a machine tool to control the position and velocity of a driven element that is driven by a servomotor. FIG. 16 is a block diagram showing a servo control section for controlling the servomotor. A servomotor 2 or a driven element 3 that is driven by the servomotor 2 is provided with velocity detecting means 5 and position detecting means 6 for detecting its velocity and position, respectively. Further, it is provided with current detecting means 4 for detecting the value of a current for driving the servomotor 2. Detected signals from the detecting means 4 and 5 are fed back.
In a position control processing section 11, a position deviation is obtained from a position command and a position feedback signal from the position detecting means 6, and a velocity command is obtained by multiplying the position deviation by a position loop gain. In a velocity control processing section 12, moreover, a velocity deviation is obtained from a velocity command outputted from the position control processing section 11 and a velocity feedback signal from the velocity detecting means 5, and a current command is obtained by velocity feedback control such as proportional plus integral (or integral plus proportional) control. In a current control processing section 13, current feedback control is performed using the current command and a current feedback signal, and the servomotor 2 is drivingly controlled with the aid of a servo amplifier.
In the above-described conventional control method of controlling the position, velocity, and current of the driven element 3, e.g., a feed axis of a machine tool, the position, velocity, and current are normally controlled by processors. If the angular acceleration of the servomotor 2 changes suddenly, the driven element 3 sometimes may vibrate despite the position, velocity, and current feedback controls. To cope with this, a control method is proposed in which a signal from an acceleration sensor for detecting the acceleration of the driven element 3 is subtracted from a current command outputted by the velocity feedback control, and the resulting difference is used as a current command for the current feedback control.
If vibration is generated in the driven element, a vibration component in an acceleration signal from the driven element detected by the acceleration sensor causes an error against the current command for the current feedback control. Therefore, the vibration is restrained by controlling the driving current of the servomotor with the vibration component subtracted from the current command to eliminate the error (see Jpn. Pat. Appln. KOKAI Publication No. 6-91482).
It is known that response can be improved by differential control in a control system. Also in position/velocity control of the servomotor, the response can be enhanced by using PID (proportional plus integral plus derivative) control for velocity loop control. However, acceleration information that is obtained by differentiating position or velocity information from a position or velocity sensor (or by differentiating the velocity deviation) has problems of much noise and poor controllability. If a filter is additionally used to remove noise, moreover, phase change that is caused by the filter inevitably worsens the controllability.
Acceleration information with less noise and phase change can be obtained with used of an acceleration sensor such as the one described in JP 6-91482A. According to the invention described in this patent document, however, the current command is corrected for current control processing based on a detected acceleration value detected by the acceleration sensor.
In general, a current control period is shorter than a velocity control period, so that processing time increases if the current command is corrected with the detected acceleration value.
In a method where a detected acceleration value is subtracted from a current command value, moreover, a satisfactory vibration restraint effect sometimes cannot be obtained, owing to a delay in communication for capturing the detected acceleration value into a controller or a delay from the current command input to the start of movement of the driven element. Furthermore, there is a problem that the integral of the acceleration information fails to become zero when the driven element returns to its original position after reciprocation under correction control of the current command with the detected acceleration value detected by the acceleration sensor.